In many applications, reliable and accurate measurements of high frequency voltage and current may be needed. Such applications may include, for example, RF plasma generation and RF plasma etching. A typical RF plasma generator may include a high power RF source that generates RF signals at a given frequency (for example 13.56 MHz). The RF signals may be supplied to a plasma chamber. An impedance matching network may be provided between the RF power source and the plasma chamber, due to the considerable impedance mismatch that may exist between the RF power source and the plasma chamber. A VI (voltage-current) sensor or probe may be provided to detect the voltage and current of the RF signals as they enter the plasma chamber, and to generate sensor signals representative of the detected voltage and current.
The magnitude and phase of these high-frequency voltage and current measurements may have to be obtained accurately and repeatably. Digital operations (such as mixing and filtering) that are used to extract magnitude and phase information from the sensor signals may typically be performed using oversampling techniques, i.e. by sampling the data at frequencies equal to or higher than twice the highest frequency of interest, in accordance with the Nyquist criterion.
Oversampling techniques, however, may not accurately reconstruct the measured sensor signals, which may include multiple harmonics of a fundamental frequency, as well as magnitude and phase noise. Also, it may not be possible to implement oversampling techniques at microwave frequencies, for example 2.4 GHz, because at microwave frequencies the sampling frequency may have to be at least about 4.8 Gsps in order to satisfy Nyquist criterion. Currently available high-speed A/D (analog-to-digital) converters may only have sampling frequencies in the range of about 105 to about 400 Msps.
Fixed sampling frequencies may be used, for either oversampling or undersampling, when the analog signals from the VI sensor are sampled. These fixed sampling frequencies may be asynchronous with time-changing RF frequencies. The RF signals from the RF source (as well as the sensor signals) may have time-varying frequencies, however, as these signals are tuned in order to controllably adjust the impedance and to control the power delivered to the plasma chamber.
If a signal whose frequency changes with time is sampled at fixed frequencies, i.e. asynchronously, the signal measurements may degrade due to jitters or resonance. If measurements with jitter or resonance effects are used in high-speed closed loop control, the system performance may degrade quite dramatically.